1. Field of the Invention
The present invention relates to the field of power dimmers which are intended to adjust the A.C. supply power of resistive or capacitive loads.
2. Discussion of the Related Art
A first known category of dimmers is based on the use of triacs, the triggering delay of which is varied according to the beginning of the halfwaves of the A.C. power supply. Such dimmers do not work with capacitive loads.
A second category to which the present invention applies relates to dimmers using switches, the turn-off delay of which is controlled and which are thus on at the beginning of each halfwave of the power supply. Such dimmers require lockable switches, for example, two MOS transistors respectively assigned to the positive and negative halfwaves of the A.C. power supply. Such dimmers require, for each of the MOS transistor control amplifiers, dedicated supply circuits, which increases the circuit cost. Further, the use of MOS transistors poses electro-magnetic compatibility problems as well as forward voltage problems when the two transistors are in anti-series.
To solve this last problem, it has already been provided to use another category of switches, that is, so-called MBS transistors which correspond to IGBT transistors holding the reverse voltage (reverse blocking IGBT).
FIG. 1 shows a conventional example of such a dimmer. A load (for example, a lamp L) is connected in series with a power dimmer 1 between two terminals P and N of application of an A.C. supply voltage. Circuit 1 is based on the use of two MBS transistors 2 and 3 connected in antiparallel between two terminals 4 and 5 of the dimmer, respectively connected to lamp L and to terminal N. In this example, transistor 2 has its emitter on the side of terminal 4 and transistor 3 has its emitter on the side of terminal 5. Each MBS is controlled by a circuit 6, 7 (DRIV) specific thereto. Such drivers 6, 7 generally comprise a comparator associated with a detector of the zero crossing of the A.C. voltage, connected to delay the MBS blocking with respect to the zero crossing according to a reference value. The power reference value is provided, for example, by a potentiometer 11 on the side of circuit 7. This reference is transferred from circuit 7 to circuit 6 by an optocoupler 10, which is necessary due the reference voltage difference between circuits 6 and 7.
Each circuit 6, respectively 7, is supplied by a circuit (PSC) 8, respectively 9. Such circuits 8 and 9 are known, for example, under trade name VIPER. These are integrated circuits or circuits with discrete elements which, in the connection of FIG. 1, are each associated with a capacitor C8, respectively C9, across which the supply voltage intended for driver 6, respectively 7 is sampled. Circuit 8 further comprises, in series between the electrode of capacitor C8 opposite to terminal 4 and terminal N, a resistor R8, a MOS transistor T8, and a diode D8. Circuit 9 comprises, between the electrode of capacitor C9 opposite to terminal N and terminal 4, a resistor R9, a MOS transistor T9, and a diode D9. Such connections based on VIPER circuits are known.
A disadvantage of a structure such as illustrated in FIG. 1 is that it requires two separate complex supply circuits for circuits 6 and 7.